The present invention relates generally to combustion turbine rotor blades and more particularly to an improved tip structure for a cooled turbine rotor blade.
It is well established that greater operating efficiency and power output of a combustion turbine may be achieved through higher inlet operating temperatures. Inlet operating temperatures are limited, however, by the maximum temperature tolerable to the rotating turbine blades. Also, as turbine blade temperature increases with increasing inlet gas temperature, the vulnerability of the blades to damage from the tension and stresses which normally accompany blade rotation increases. Cooling the turbine blades, or forming the turbine blades from a temperature resistant material, or both, permits an increase in inlet operating temperatures while keeping turbine blade temperature below the maximum specified operating temperature for the blade material.
In a typical prior art combustion turbine, cooling air drawn from a compressor section of the turbine is passed through channels in the turbine rotor to each of several rotor discs. Passageways within each rotor disc communicate the cooling air from the turbine rotor to a blade root at the base of each turbine blade. Generally, the cooling air flows from the blade root through an airfoil portion of the blade and exits at least partially through the tip of the blade.
A typical prior art blade tip structure defines an outwardly facing cavity formed by a radially outward extension of the blade wall surrounding the exterior surface of the blade tip. Cooling air exits from apertures in the exterior surface of the blade tip into the cavity. The tip cavity structure prevents sealing of individual exhaust apertures by a minor contact between the blade tip and the surrounding turbine casing. Such a blockage, or blade tip smear, could result in burning of the turbine blade due to reduced cooling air flow through the blade. The prior art includes two different blade tip cavity structures, the choice of structure depending upon the blade row in which the blade is positioned. Generally, the blade geometry varies with each row of turbine blades.
One geometric variable is the thickness of the turbine blade trailing edge the thickness typically decreasing by row in the downstream direction. In initial turbine blade rows the trailing edge is thick enough to support an extension of the blade wall so that the blade tip cavity extends over the trailing edge to cover the entire exterior blade tip surface. In this configuration all apertures in the exterior blade tip surface vent cooling air into the cavity. A portion of the blade wall toward the trailing edge of a convex side of the blade is removed to provide a cooling air exit path from the blade tip cavity. This structure is described in greater detail in Swiss Pat. No. 225,231 and U.S. Pat. No. 3,635,585.
In downstream blade rows, where the thickness of the trailing edge becomes too thin to support an extension of the blade wall, the blade tip cavity must terminate at some point short of the trailing edge of the blade. With no cavity to protect the apertures in the blade tip surface at the trailing edge, an alternate means must be devised to prevent the apertures outside the cavity from being sealed by a blade tip smear.
In typical prior art, a window or notch is structured in the concave side of the trailing edge of the blade so that the cooling air exits from apertures which are recessed from the radially outermost point on the blade tip surface. The window in the trailing edge effectively prevents the exhaust apertures therein from being closed by a blade tip smear, but does so at a cost to the efficiency of the turbine blade. The window removes a portion of the working surface on the concave side of the blade, thereby reducing blade efficiency.
It would be advantageous to design a turbine blade with tip structure at the trailing edge which effectively prevents closure of cooling air apertures outside the tip cavity by blade tip smearing but does not detract from turbine blade efficiency by removal of a portion of the blade wall.